I know your pain, I've been there before!
Firstly, buy new bolts, as even if you get that one out intact, you won't want to re-use it! When I had this problem (on a Subaru Forester), I got a kit that had new bolts, slide pins and gaiters, and replaced the lot.
You've then got a few options...
You've tried WD-40, but that's not always the best thing - you need a proper penetrating oil. Spray on, leave, repeat several times.
You can then try a tool called a 'speed wrench' - this looks a bit like an adjustable spanner, but with curved, ridged jaws - the idea is that as you try to turn it, it tightens itself up and the jaws dig into the bolt head.
If that doesn't work, the next option is a bolt extractor set. These look like a drill bit with a reverse, tapered thread - you drill a hole in the end of the bolt, insert the bit and turn it in the 'undo' direction - and as you turn, it tightens itself into the hole. I tried one of these and snapped it off in the hole!
Failing that, you're probably best cutting the head of the bolt off... Depending on the size, you might be able to use a Dremel or similar. I ended up resorting to a grinding disk in an angle grinder, and very slowly and carefully grinding down the head until it released - making sure not to cut into the caliper bracket. Once I'd got the head off, I could dismantle the assembly and was able to remove the remains of the bolt from the slider with locking pliers (though as I mentioned above, I actually used new sliders in the end, as the old ones were corroded)
In my case, it was only one bolt that wouldn't come out, as a previous repairer had used the wrong bolt - it was clearly different to all the others...